(a) Field of the Invention
The present invention concerns a transistor amplifier circuitry for amplifying a low level input signal with a high S/N ratio, and also pertains to a low-noise transistor intended for low level input signal which is most suitable as an amplifying element for use in the input stage circuit of such transistor amplifier circuitry.
(B) Description of the Prior Art
Known transistor amplifier circitry intended for low level input signal and required to have a good S/N ratio, is arranged to be operative in such a way as to minimize noises produced from the transistors which constitute the amplifier circuitry by appropriately setting the operating conditions of these transistors. More particularly, in such a known amplifier circuitry, the emitter current as well as the collector current of the component transistors are set at as small a value as possible. However, in a transistor, the values of the collector current and the emitter current both exert not a small influence upon such properties of the amplifier circuitry as the current amplification factor and the cut-off frequency. Accordingly, it is not easy to realize an optimum S/N ratio by relying on such known techniques as those mentioned above. Moreover, even when the collector current and the emitter current of the transistors are set at such values as will cause the S/N ratio to be at the optimum condition, it still is not possible to improve this S/N ratio to such an extent as is expected and desired.
After extensive researches and experiments, the inventor has discovered that such problems as discussed above encountered in the known transistor amplifier circuitries for low level signal are attributable essentially to the properties per se of the known transistors designed for low input signals. More particularly, let us now refer to FIG. 1. The noise equivalent circuit of the transistors is expressed in a manner as shown in FIG. 1. In this figure, symbols B, C and E represent the base electrode, the collector electrode and the emitter electrode, respectively. Symbols r.sub.bb, r.sub.c and r.sub.e represent the base spreading resistance, the collector resistance and the emitter resistance, respectively, based on the assumption that none of these resistances will cause noises. Symbols i.sub.ne and i.sub.nc represent equivalent current sources of noises which are produced from the emitter portion and the collector portion, respectively. Also, symbol v.sub.nb represents an equivalent voltage source of noises produced from the base spreading resistance r.sub.bb. Thus, under the assumption that the above-mentioned noises from the above-mentioned respective noise sources contain no excess noises (flicker noises), these noises are expressed by the following equations:
d EQU i.sub.ne = .sqroot.2q(I.sub.E + 2I.sub.ES).DELTA.f Eq. 1 EQU i.sub.nc = .sqroot.2q I.sub.C .DELTA.f Eq. 2 EQU v.sub.nb = .sqroot.4kT r.sub.bb .DELTA.f Eq. 3
wherein:
k represents a Boltzmann constant; PA1 T represents an absolute temperature; PA1 .DELTA.f represents the width of the noise equivalent band; PA1 q represents the charge on a single electron; PA1 I.sub.e and I.sub.C represent the emitter current and the collector current, respectively; and PA1 I.sub.es represents the saturation inverse current at the emitter junction.
As will be clear from the above equations, in the known transistor amplifier circuitry intended for low level signal, it is arranged so that the emitter current I.sub.E and the collector current I.sub.C of the component transistors, especially of the transistor provided on the input stage, are set to have minimal values to reduce both the noises i.sub.ne produced from the emitter portion and the noises i.sub.nc produced from the collector portion of the transistor. However, the transistor of the prior art intended for low level signal is such that its base spreading resistance r.sub.bb has a value distribution ranging from several scores of ohms to several hundreds of ohms. Therefore, even where a device for a reduction of noises i.sub.ne and i.sub.nc in a manner described above is incorporated in the prior art such circuitry, there are developed thermal noises v.sub.nb of a great magnitude which come from the high base spreading resistance r.sub.bb. Thus, no sufficient improvement of the S/N ratio in amplifier circuitries is attained. This fact has been elucidated by the inventor as a result of his extensive series of experiments and researches conducted for an extended period of time on prior art transistors intended for low level input signals.
According to the observation of the inventor, the reasons why the above-said shortcomings of the known transistors for low level signals have not been improved are considered to be related to an erroneous concept in the designing of the conventional transistor amplifier circuitries intended for low level signals. Regarding this point, explanation will be made hereunder by referring to FIG. 2.
FIG. 2 shows a general arrangement of a transistor amplifier circuitry for low level signal. In this figure, symbol A represents a phase-inverting amplifier circuit. R.sub.f represents a resistor for feeding, back to the input side, the output of this phase-inverting amplifier circuit. R.sub.i represents a resistor for connecting the input terminal P to the input terminal IN, of the phase-inverting amplifier circuit A. Symbol R.sub.g represents a true input resistance of the phase-inverting amplifier circuit A. Assuming that this phase-inverting amplifier circuit A has a sufficiently large open loop gain, the apparent input impedance Z.sub.i as viewed from the input terminal IN and the gain G of the transistor amplifier circuitry for low level signal having the afore-mentioned arrangement will be expressed by the following equations: ##EQU1## EQU Z.sub.i .apprxeq. R.sub.i Eq. 5
Now, the true input resistance R.sub.g of the phase-inverting amplifier circuit A constitutes the principal noise source of this amplifier circuit A, and accordingly, of the transistor amplifier circuitry as a whole intended for low level signal. In order to attain a high S/N ratio of such a circuit of circuitry, it is necessary to minimize the value of said true input resistance R.sub.g. Especially, in case the impedance of the signal source which is connected to the circuit or circuitry is low, the S/N ratio will be markedly lowered due to the thermal noises which are produced from the true input resistance R.sub.g.
However, as will be understood clearly from Eq. 5 the apparent input impedance Z.sub.i is determined substantially by the value of the resistor R.sub.i. Therefore, in the designing, in the past, of the transistor amplifier circuitry for low level signal, there has been hardly paid any consideration to the reducing of this true input resistance R.sub.g.
It should be noted here that, in case the input stage circuit of the phase-inverting amplifier circuit is constructed either by the emitter-grounded circuit or by the base-grounded circuit of a transistor, the true input resistance R.sub.g is expressed by the following equation: ##EQU2## wherein; h.sub.ie, h.sub.fe, r.sub.bb, r.sub.e and I.sub.E represent the output short circuit input impedance, the output short circuit forward-current transfer ratio, the base spreading resistance, the forward direction resistance at the emitter junction, and the emitter current, respectively.
As will be clear from Eq. 6, in order to minimize the true input resistance R.sub.g, the base spreading resistance r.sub.bb of the transistor has to be minimized. However, the conventional designing concept lacks the aforesaid concept of making the true input resistance R.sub.g small. Accordingly, in the conventional transistor intended for low level signal, there is hardly given any efforts to the reduction of the base spreading resistance r.sub.bb.